The invention is based on a priority application EP03360055.2 which is hereby incorporated by reference.
The present invention relates to a network element for use in an optical communication network and in particular to a network element for use in a DWDM communication network.
The invention further relates to an optical communication network comprising corresponding network elements.
Optical communication networks and in particular DWDM (Dense Wavelength Division Multiplex) communication networks are basically known. For instance, U.S. Pat. No. 6,307,986 B1 discloses an approach for protection switching in bidirectional WDM optical communication networks using transponders. The background portion of this document provides a general overview of such kind of networks and refers to further background literature.
A basic idea of a DWDM communication network is to transmit a plurality of optical communication signals across a common optical fiber by using wavelength division multiplexing. This means that the plurality of messages to be transmitted is modulated onto a plurality of optical carrier wavelengths thereby providing wavelength separated communication channels for each message.
The modulation process may be carried out using various modulation techniques and parameters. Each of the various alternatives is referred to as a modulation scheme in the following. The modulation schemes may comprise different modulation formats, such as Amplitude Shift Keying with Return-to-Zero signals (RZ), Amplitude Shift Keying with Non-Return-to-Zero (NRZ), Phase Shift Keying and in particular Differential Phase Shift Keying (DPSK), Phase-Shaped Binary Transmission (PSBT), and others. The modulation schemes may further comprise different modulation techniques, such as direct modulation of a laser source or external modulation of a continuous light beam provided from a light source. Likewise, the optical network may be designed to communicate message signals with a specific bit rate in all the communication channels.
As it is known to the skilled person in the field of optical communication technology, optical signals propagating along an optical fiber suffer from a plurality of different signal impairments, in particular from chromatic dispersion, polarization mode dispersion, scattering and others. Since these signal impairments become more severe the longer the spans between an optical transmitter and an optical receiver is, it is difficult to scale up an existing optical communication network designed for covering short or medium distances to long haul or ultra-long haul hops. Moreover, since the signal impairments are also dependent on the wavelength used, it is also difficult to add or switch to new wavelengths in an existing concept.
Previous approaches to cope with the increasing problems of signal impairments in long distance optical networks have envisaged the introduction of dispersion compensation devices, preferably adaptive compensation devices. While it is basically possible to increase the signal quality transmitted over long distance hops, such compensation devices are rather expensive thereby increasing the costs of the overall network concept. Additionally, there may be circumstances where such adaptive compensation devices introduce instabilities in the network.
On the other hand, it is known to those skilled in the art that different modulation schemes, and in particular different modulation formats, might be differently affected by the signal impairments. In other words, a PSBT transmission, for instance, is more robust against dispersion than NRZ, for instance. A successful approach using PSBT is disclosed, for instance, by Charlet et al., “Capacity over 21×100 km Using Bandwidth-Limited Phase-Shaped Binary Transmission”, ECOC′02. Likewise, a RZ modulation format is interestingly robust against polarization mode dispersion, cf. Khosravani et al., “Comparison of Different Modulation Formats in Terrestrial Systems with High Polarization Mode Dispersion”, OFC′00, WL5.